Tutorial :Possible to implement a manual increment with just simple SQL INSERT?

Question:

I have a primary key that I don't want to auto increment (for various reasons) and so I'm looking for a way to simply increment that field when I INSERT. By simply, I mean without stored procedures and without triggers, so just a series of SQL commands (preferably one command).

Solution:4

We have a similar situation where we needed to increment and could not have gaps in the numbers. (If you use an identity value and a transaction is rolled back, that number will not be inserted and you will have gaps because the identity value does not roll back.)

We created a separate table for last number used and seeded it with 0.

Our insert takes a few steps.

--increment the number Update dbo.NumberTable set number = number + 1

--find out what the incremented number is select @number = number from dbo.NumberTable

--use the number insert into dbo.MyTable using the @number

commit or rollback

This causes simultaneous transactions to process in a single line as each concurrent transaction will wait because the NumberTable is locked. As soon as the waiting transaction gets the lock, it increments the current value and locks it from others. That current value is the last number used and if a transaction is rolled back, the NumberTable update is also rolled back so there are no gaps.

Hope that helps.

Another way to cause single file execution is to use a SQL application lock. We have used that approach for longer running processes like synchronizing data between systems so only one synchronizing process can run at a time.

Solution:5

It could be because there are no records so the sub query is returning NULL...try

Solution:8

But perhaps a better approach would be using a scalar function getNextId('table1')

Solution:9

It seems very odd to do this sort of thing w/o an IDENTITY (auto-increment) column, making me question the architecture itself. I mean, seriously, this is the perfect situation for an IDENTITY column. It might help us answer your question if you'd explain the reasoning behind this decision. =)

Having said that, some options are:

using an INSTEAD OF trigger for this purpose. So, you'd do your INSERT (the INSERT statement would not need to pass in an ID). The trigger code would handle inserting the appropriate ID. You'd need to use the WITH (UPDLOCK, HOLDLOCK) syntax used by another answerer to hold the lock for the duration of the trigger (which is implicitly wrapped in a transaction) & to elevate the lock type from "shared" to "update" lock (IIRC).

you can use the idea above, but have a table whose purpose is to store the last, max value inserted into the table. So, once the table is set up, you would no longer have to do a SELECT MAX(ID) every time. You'd simply increment the value in the table. This is safe provided that you use appropriate locking (as discussed). Again, that avoids repeated table scans every time you INSERT.

use GUIDs instead of IDs. It's much easier to merge tables across databases, since the GUIDs will always be unique (whereas records across databases will have conflicting integer IDs). To avoid page splitting, sequential GUIDs can be used. This is only beneficial if you might need to do database merging.

Use a stored proc in lieu of the trigger approach (since triggers are to be avoided, for some reason). You'd still have the locking issue (and the performance problems that can arise). But sprocs are preferred over dynamic SQL (in the context of applications), and are often much more performant.